The present invention relates to a ferritic heat resisting steel having superior high-temperature strength and suitable for use as material of parts of power-generating steam and gas turbines, particularly, turbine blades, turbine disks and bolts used in such turbines.
In recent years, there is a trend for higher operation temperature and pressure of steam power-generating stations for the purpose of attaining higher efficiency. For instance, steam temperature in gas turbines, which is 566.degree. C. at the highest in existing turbines, is planned to be elevated to 600.degree. C. and finally to 650.degree. C. A higher steam temperature essentially requires heat resisting materials which are superior in high-temperature strength to conventionally used ferritic heat resisting steels. Some of austenitic heat resisting alloy steel exhibit superior high-temperature strength. These heat-resisting alloy steels, however, cannot be put to practical use partly because of inferior thermal fatigue strength due to large thermal expansion coefficient and partly because of high price.
Under these circumstances, there have been proposed many ferritic heat resisting steels having improved high-temperature strength. Examples of such strength. Throughout the study, the present inventors have found that the high-temperature strength is further improved by multiplied effect of W and Co, when Co content is positively increased as compared with the alloys of the same type while adding Mo and W simultaneously with the W content increased as compared with the known alloys. The present invention is based upon this discovery.
According to one aspect of the present invention, there is provided a ferritic heat resisting steel having a composition containing, by weight, 0.05 to 0.20% C (carbon), 0.05 to 1.5% Mn, 0.05 to 1.0% Ni, 9.0 to 13.0% Cr, 0.05 to less than 0.50% Mo, 2.0 to 3.5% W, 0.05 to 0.30% V, 0.01 to 0.20% Nb, 2.1 to 10.0% Co, 0.01 to 0.1% N, and the balance substantially Fe and incidental impurities, with Si as an impurity limited to be not more than 0.15%. Part of Fe may be substituted by 0.001 to 0.030% of B.
According to another aspect of the present invention, there is provided a ferritic heat resisting steel having a composition containing, by weight, 0.09 to 0.13% C, 0.3 to 0.7% Mn, 0.3 to 0.7% Ni, 9.0 to 13.0% Cr, 0.1 to 0.2% Mo, 2.4 to 3.0% W, 0.15 to 0.25% V, 0.05 to 0.13% Nb, 2.1 to 4.0% Co, 0.02 to 0.04% N, and the balance substantially Fe and incidental impurities, with Si as an impurity limited to be not more than 0.15%. Part of Fe may be substituted by 0.001 to 0.030% of B.
According to still another aspect of the invention, there is provided a ferritic heat resisting 1 steel having a composition containing, by weight, 0.10 to 0.12% C, 0.35 to 0.65% Mn, 0.4 to 0.6% Ni, 10.8 to 11.2% Cr, 0.1 to 0.2% Mo, 2.5 to 2.7% W, 0.15 to 0.25% V, 0.05 to 0.11% Nb, 2.7 to 3.1% Co, 0.02 to 0.03% N, 0.01 to 5 0.02% B, and the balance substantially Fe and incidental impurities, with Si as an impurity limited to be not more than 0.10%.